The invention relates to an exhaust system for an internal-combustion engine as well as to a method of operating the exhaust system.
From German Published Patent Application DE 10 2009 032 213 A1, an exhaust system of an internal-combustion engine is known, for example, which has a first exhaust gas system assigned to a first cylinder group of the internal-combustion engine and has a second exhaust gas system assigned to a second cylinder group of the internal-combustion engine. Each exhaust gas system has one emission control device respectively, as well as a first muffler arranged behind the respective emission control device, a second muffler arranged behind the respective first muffler, and an exhaust gas tailpipe arranged behind the respective second muffler.
It is a disadvantage of this known state of the art that the damping resonator of the muffler cannot be variably adjusted with respect to sound.
Furthermore, mufflers for exhaust systems are generally known from the state of the art, which muffles operate according to the absorption and/or reflection principle. The development of such a muffler, as a rule, involves finding the best-possible compromise between the muzzle noise (loudness after the exhaust system tailpipe), the exhaust backpressure and the required muffler volume. For bypassing such a compromise solution, mufflers are frequently designed with one or more movable closing elements or shut-off devices, such as an exhaust flap, in order to provide different flow paths in the exhaust pipe of the muffler system. In the case of muffler systems for internal-combustion engines, often two, three or even four exhaust system tailpipes are provided. In muffler constructions known from prior art, the arrangement of the closing elements has the disadvantageous effect that, depending on the position of the closing element, exhaust gas will no longer flow through all exhaust system tailpipes or even tailpipe branching elements.
This state of the art has the following disadvantages:                1) Irritation on the part of the customer because exhaust gas does not flow out of all exhaust system tailpipes (at low outside temperatures recognizable by water vapor and by the different degree of dirt at the visible tailpipes);        2) High exhaust gas backpressure as a result of a bottleneck in the exhaust system tailpipe;        3) Audible flow-generated noise as a result of the bottleneck in the exhaust system tailpipe;        4) Thermal stress between the cold and hot exhaust system tailpipe carries the risk of crack formation;        5) Costly guidance of the damping pipes in the case of the muffler system;        6) Heavy weight of muffler system;        7) High manufacturing costs of muffler system; and        8) Normally, flaps are to be separately switched only to a limited extent and in a time-staggered manner. This results in a clear rise of the exhaust gas backpressure in an exhaust gas system.        
It is an object of the present invention to avoid the above-mentioned disadvantages and simultaneously introduce variability into the acoustic damping characteristics of the muffler system.
This and other objects are achieved by providing an exhaust system for an internal-combustion engine, having at least a first and a second cylinder, a first exhaust pipe being assigned to the first cylinder and a second exhaust pipe being assigned to the second cylinder, and a first muffler being assigned to the first exhaust pipe and a second muffler being assigned to the second exhaust pipe. A first damping pipe branches off the first exhaust pipe upstream of a first shut-off element, which first damping pipe first leads into a first reflection chamber and subsequently extends through the first muffler and leads into the second exhaust pipe downstream of a second shut-off element. A second damping pipe branches off the second exhaust pipe upstream of a second shut-off element, which second damping pipe first leads into a second reflection chamber and subsequently extends through the second muffler and leads into the first exhaust pipe downstream of the first shut-off element.
As a result of the further development of the exhaust system according to the invention, all above-mentioned problems will be avoided. The exhaust system has a construction that is advantageous with respect to the exhaust gas backpressure. By means of the construction according to the invention, one or both shut-off elements can also be closed simultaneously without significantly increasing the exhaust gas backpressure. In contrast to conventional exhaust systems, when the shut-off elements, such as the exhaust gas flaps, are open, a minimal or hardly measurable throttling (rise of the exhaust gas backpressure) will take place, because the entire exhaust pipe volume and damping pipe volume is utilized to the end of the exhaust system. By way of a slightly altered construction, the shut-off elements can also be switched in a time-staggered manner, in order to improve a subjective hearing impression during the switching phases. As a result of the staggered switching-over of the two shut-off elements, noticeable acoustic level jumps can be advantageously reduced.
With respect to performance and dynamics (response behavior of the internal-combustion engine), the very low exhaust gas backpressure of the exhaust system caused by the further development according to the invention has a very positive effect. For slight acoustic adaptations, the volumes of the first muffler and of the second muffler can also be changed later. In addition, one or more possibly required resonators can be integrated in the exhaust system without large expenditures and while retaining symmetry. As a result of the symmetrical construction of the exhaust system, the latter and furthermore the muffler itself can have a very advantageous structure.
Many components of the exhaust system according to the invention can be produced or used in a cost-effective manner as identical parts.
The muffler housing may, for example, be implemented as a cost-effective wound muffler or in a shell construction. When it is implemented as a wound muffler, the two lateral parts can be constructed as identical parts. In the shell construction, the top shell and the bottom shell can be designed of identical parts respectively.
The exhaust pipes can also be produced and installed as identical parts.
The Y-branching elements from the exhaust pipes to the damping pipes may, for example, be implemented as an internal high-pressure preform (IHU). The damping pipes with perforations, which extend through the mufflers, can also be implemented as identical parts. This also applies to the damping pipe from the Y-pipe piece into the reflection chamber.
Because of the identical parts for the exhaust system, tools can also be saved for the assembly. Depending on how a manufacturer's or supplier's production is structured, an identical assembly system can be used for the pre-assembly of the Y-branching elements and of the shut-off elements for both sides of the exhaust system.
All above-mentioned points result in a significant reduction of the tool and production costs for the exhaust system according to the invention.
In addition, as a result of the large proportion of identical parts and the reduction of component variants, a possibly desired light-weight construction can clearly be implemented more easily.
In a further development, a third reflection chamber is assigned to the first exhaust pipe downstream of the first shut-off element, and/or a fourth reflection chamber is assigned to the second exhaust pipe downstream of the second shut-off element.
In a still further development, the first exhaust pipe is connected by way of perforations in an exhaust-gas-carrying manner with the third reflection chamber, and/or the second exhaust pipe is connected by way of perforations in an exhaust-gas-carrying manner with the fourth reflection chamber.
A further fine-tuning of the exhaust system sound can be achieved by way of a further development in that the first and the third reflection chambers and/or the second and the fourth reflection chambers are connected with one another in an exhaust-gas-carrying manner. In order to reduce manufacturing costs even more, in a further development the first and second mufflers are arranged in a common housing. Moreover, the first and second reflector chambers may he arranged in the housing, and the third and fourth reflection chambers may be arranged in the housing.
Minimal damping is achieved by means of the method of operating an exhaust system for an internal-combustion engine wherein the first and second shut-off elements are opened.
Medium damping is achieved by means of the method of operating an exhaust system for an internal-combustion engine wherein the first shut-off element is opened and the second shut-off element is closed.
Maximal damping is achieved by means of the method of operating an exhaust system for an internal-combustion engine wherein the first and second shut-off elements are closed.
In the following, an exhaust system according to the invention and three methods of operating the exhaust system according to the invention will be explained.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.